This is an irregular verb. It follows the conjugation pattern of the verb leer.
Accent Change: i í (creí, creído)
Ending Change: i y (creyó)
Gerund: creyendo
PresentPresente
Yo creo que mañana va a llover.
I think it is going to rain tomorrow.
PreteritePretérito indefinido
Nadie creyó su historia del lunes.
Nobody believed his story from Monday.
ImperfectPretérito imperfecto
De pequeño, creía en los fantasmas.
As a kid, I believed in ghosts.